A surface acoustic wave apparatus, in particular a surface acoustic wave filter has found a wide use for a RF band-pass filter in a mobile telephone and/or a handy telephone, etc. in place of a dielectric band-pass filter. The reasons for that may be that surface acoustic wave apparatus, in particular surface acoustic wave filters are smaller in size than dielectric filters, and also shows improved electric characteristics as compared with dielectric filters of the same sizes.
A surface acoustic wave apparatus includes at least a piezoelectric substrate, an electrode pattern consisting of a metal film formed on a surface of the piezoelectric substrate, and a package accommodating the piezoelectric substrate and the electrode pattern. The piezoelectric substrate is made of a material, such as a lithium niobate, a lithium tantalate, or a quartz, an electrode pattern of a film of a metal, such as aluminum being formed thereon and accommodated into the package.
In FIG. 5, there is shown a manufacturing process of a conventional prior art surface acoustic wave apparatus. First, a piezoelectric substrate 50 is provided by being cleaned in the process (a), and a metal film 51 is formed thereon by means of a vapor-deposition or a sputtering to provide an electrode material in the process (b). A coating of a photoresist is then provided thereon, for example, by means of a spin coat method. Then, as shown in the process (c), exposure to a desired pattern is made by an exposing machine, and after development, obtain a photoresist pattern 52. Thereafter, in the process (d), a wet-etching or a dry-etching method is carried out to form a metal film of a desired electrode pattern 53. The photoresist used for the pattern formation is, in the process (e), removed using a removing agent, or by means of an ashing method. Thus, a pre-process, which is so-called photo-process, has been completed. Thereafter, in the process (f), the piezoelectric substrate formed with the electrode pattern thereon is cut into chips by dicing. Then, in the process (g), after having fixed the cut chips on the package using an adhesive, bonding wires are attached thereto in the process (h). Finally, in order to secure a hermeticity, a lid is welded to the package in the process (i), an electrical characteristic test is made in the process (j) to complete the post-process.
The surface acoustic wave apparatus has a problem such that, when in use for the RF band around 1 GHz, an electrode width and an electrode gap of a comb-shaped electrode must be of very small values, such as about 1 micrometer, thereby its life time (hereinafter, referred to as life) is very short. A factor of determining the life for the surface acoustic wave apparatus is mainly a power-durability of the electrode film. Placing the surface acoustic wave apparatus in an operating state, the electrode film on the piezoelectric substrate is subjected to a stress which is repeated with a frequency that is equal to an operating frequency. Due to this repetitive stress applied to the electrode film, defects such as hillocks (projections) and voids (depletions) will be generated on the electrode film by migrations, thereby significantly degrading the characteristics of the surface acoustic wave apparatus. This degradation phenomena of electrode film will be more pronounced for higher operating frequency, and for larger applied electric powers. Further, as a matter of design, for higher operating frequency, it is necessary to make the electrode film thinner and to make the electrode width narrower. With such additional factors, in a design for higher operating frequency, defects would be easily generated on the electrode film, and as a consequence, resulting in a poor power-durability.
As the electrode film for the surface acoustic wave apparatus, aluminum (Al) has been at first used because of its small specific gravity, its small electric resistivity, etc. However, when aluminum (Al) is used for the electrode film, problems of the above mentioned degradation of the electrode film have arisen in designs for higher operating frequency. As a means for improving the degradation phenomena of the electrode film caused by this migration, there has been proposed to use an aluminum-copper alloy (Al--Cu) formed by adding a small amount of a different kind of metal such as copper (Cu) to aluminum (Al) as disclosed by J. I. Latham, et al. (See, Thin Solid Films, 64, pp. 9-15, 1979). With this aluminum alloy, the generations of hillocks and voids can be suppressed, and the power-durability of the surface acoustic wave apparatus has been enhanced.
Meanwhile, the occurrence of degradation caused by migration of aluminum has also been a problem in the field of semiconductor. As the migration of aluminum in the semiconductor wiring, mainly two kinds are known such as an electro-migration produced by an electric current, and a stress-migration produced by a residual stress resulted from making the multiple-layers of thin films. As distinct from a circumstance in the field of semiconductor, the surface acoustic wave apparatus is subjected to a stress which is repeated with a frequency equal to an operating frequency, as mentioned above. In the article "METAL-MIGRATION OF ALUMINUM THIN FILM ON SAW RESONATOR", the Journal of Institute of Electronics, Information and Communication Engineers of Japan, Vol. J67-C, No. 3, pp. 278-285, 1984, Ebata and Morishita describe about the migration of aluminum in details. The migration of aluminum has been a problem in the semiconductor wiring at first. The migration for the semiconductor wiring is classified roughly into the electro-migration (EM) caused by a direct current and the stress-migration (SM) caused by a residual stress which has been produced in forming multiple-layers wiring. In this article, the causes of migrations produced in the surface acoustic wave apparatus are clearly explained. According to this explanation, a shield electrode is connected to so that it is grounded between two interdigital transducers (IDT) in a manner that an excitation by and receipt of the surface acoustic wave (SAW) will not be made, a streak pattern corresponding to the standing wave of the surface acoustic wave (SAW) is formed in the operation on the shield electrode. This streak pattern forms the voids and hillocks caused by the migration of aluminum. In this case, it has been verified that no current flows through the shield electrode and that a stress produced by the standing wave of surface acoustic wave is the cause for this streak. Therefore, it could be said that the migration produced in the aluminum in case of surface acoustic wave apparatus is an acoustic migration which differs from the EM, SM produced by the current or the residual stress. On one hand, since the repetitive stress corresponding to the surface acoustic wave will act on the surface acoustic wave electrode, this phenomena could be called as a metal fatigue. It is said that this stress is of a value equivalent to or greater than the tensile stress (fatigue limit) of aluminum in the form of block.
In a RF interstage filter for a handy telephone, the electric power applied to the transmitting side is different from that to the receiving side, and generally the electric power applied to the receiving side is smaller than that to the transmitting side. However, even in the case of a receiving side filter having a smaller applied electric power, it is required that its life should be more than 10 years in the room temperature 25.degree. C., with 0.1 W applied electric power. In providing this filter in the form of a surface acoustic wave apparatus, more particularly, in the form of a multiple-mode type surface acoustic wave filter, when a pure aluminum film is used as an electrode material according to a conventional technology, the life is merely a few years, thereby a specification of equal to or more than 10 years could not be satisfied.
Thus, the inventors of the present application had conducted durability tests of multiple-mode type surface acoustic wave filters wherein use is made, as the electrode material, of the aforementioned aluminum-copper alloy film (Al-0.5 wt %Cu) which is said to be a high power-durability. Samples used in the tests were selected from those which had been designed as the filters in the receiving side (the center frequency is 947.5 MHz) of the GSM system which was in a rapidly widespread use mainly in Europe. Further, during the life test, due to the defects such as voids and hillocks generated in the electrode film, there will be an increase in the film resistance of that electrode film so that there will be an increase in insertion loss. Therefore, taking a TF (Time to Failure) as a measure of the element life, the element life being defined as a time until which the insertion loss is increased by 0.5 dB, the life test was conducted by changing the applied power and the ambient temperature. As a result, under the applied power of 0.1 W and in the ambient temperature of 25.degree. C. which are the requirements in GMS, the estimated element life is about 30 years for the multiple-mode type surface acoustic wave filter which is used as the sample.
From the above mentioned results, also in the multiple-mode type surface acoustic wave filter, it has been found that a sufficient power-durability could be obtained by using the aluminum-copper alloy film (Al-0.5 wt %Cu) as the electrode material. However, because a relatively large power will be applied in the filter of the transmitting side and in the antenna duplexer, for examples, there has been a problem that a sufficient life may not be obtained with the above mentioned power-durability of the electrode film.
As means for improving the life, a study of a power-durability of an aluminum-copper alloy film has been disclosed by Tabuchi, et al, in "THE POWER AGING TEST OF SAW FILTER FOR PORTABLE TELEPHONE ANTENNA DUPLEXER", Technical Reports of Institute of Electronics and Information Communication Engineers of Japan, US 87-18. According to this report, in an aluminum-copper alloy film, the life will be improved by increasing copper concentration. Further, such improvement of the life can be observed as the copper concentration is increased up to 2.5 wt %, but on the contrary, the life will be shortened by doping the copper beyond the value. In this manner, it is known that electrode materials consisting of an aluminum-copper alloy film show the most significant power-durability with 2.5 wt % concentration of copper. However, an increase in the copper concentration will cause a problem in manufacture on the grounds described below.
First, it has been well known that a large galvanic local cell will be formed with a combination of an aluminum and a copper, so that this local cell will tend to accelerate a corrosion of the electrode film during a manufacturing process, in particular, in the photolithography, etching, etc. Further, presently, in the etching, a dry-etching using a chlorine plasma gas is commonly utilized since it can form a very small electrode with high accuracy. When applying a dry-etching technique to this aluminum-copper alloy film, problems are encountered in that the etching is difficult to be carried out because of a high boiling point of chlorides of copper and that electrode corrosion is apt to be generated after the dry-etching because of possible residual chlorides of copper.
In FIG. 7, there are illustrated experimental results of the electrode corrosion occurrence which has been generated after the dry-etching using a chlorine gas plasma. In FIG. 7, the abscissa indicates copper concentration in the aluminum-copper alloy film, and the ordinate indicates the number of corrosions generated within an area of 25 micrometers.times.25 micrometers. As can be seen from the figure, the electrode corrosion, which will not be generated in a pure aluminum film, becomes more pronounced by alloying it with the copper. It should be noted that up to 0.5 wt % of the copper concentration the electrode corrosion is hardly generated, but beyond that value, it will increase exponentially with the doped amount of the copper. That is to say, increasing the doped amount of copper in order to enhance the power-durability of the aluminum-copper alloy film will in turn make the problem of the electrode corrosion more serious during the manufacturing process.
On one hand, as another means for enhancing the power-durability of the aluminum-copper alloy film, reference may be made to the three-layer laminated structure electrode which is disclosed in the Japanese Patent Application Public Disclosure No. 7-122961. This publication discloses a structure having a pair of aluminum films with a copper layer inserted therebetween. According the teaching of this publication, the power-durability can significantly be improved by means of the three-layer laminated structure comprised of an aluminum-copper alloy film-copper-an aluminum-copper alloy film, as compared with the conventional aluminum-copper alloy film. However, with this three-layer structure, although the power-durability can be significantly improved, there is a problem regarding an ease of manufacture. In this structure, since the copper exists in the mid-layer, it is apparent that the above mentioned problem of the electrode corrosion due to aluminum and copper will become more serious than the one in the aluminum-copper alloy film. Further, for aluminum copper, etchants to be used in the etching, i.e., the compositions of the etching liquid for wet-etching or the type of etching gas for dry-etching must be different, so that for producing the three-layered structure, an apparatus will be very extensive. Also in an electrode material filming, it must go through the filming processes of forming the three-layers so that there is a problem that the manufacturing cost will be more expensive.